When compared with the recent tremendous increase in our knowledge about the mechanism of digestion and absorption of fat and carbohydrate, information about the assimilation of protein is relatively rudimentary. Furthermore the effect of disease processes on protein digestion and absorption is virtually unknown. After dietary protein is hydrolyzed to small peptides by pancreatic proteases, the intestinal columnar surface cell is assigned the task of either absorbing the peptide intact or else of hydrolyzing it at the intestinal cell-lumen interface to amino acid products which can then be transported. In general, it is believed that quantitative dipeptide entry followed by intracellular hydrolysis is the principal role played by the intestine in protein absorption. However, the brush border of the intestinal cell contains oligopeptidases having very different characteristics and possessing ten times the specific activity of those in the intestinal cell sap. We propose to characterize these brush border peptidases in both rat and man, after solubilization with papain-cysteine, by gel filtration chromatography, density gradient centrifugation, DEAE- Sephadex chromatography and preparative disc gel electrophoresis in order to determine the number of peptidases and their peptide specificity. Peptidase activity will be specifically assayed by use of L-amino acid oxidase to quantify the amino acids released. After complete purification of each of these oligopeptidases from rat brush border, the enzyme will be used to stimulate a specific anti- peptidase antibody in rabbits. This precipitating antibody will be useful in studying the synthesis and degradation of the oligopeptidase in vivo since it will allow specific immunoprecipitation of C-14 labeled enzyme from a crude solution of proteins after parenteral administration of an amino acid C-14 precursor into the intact rat. By altering the quantity and type of food in the diet, it should be possible to determine the control mechanisms that regulate these enzymes. Further experiments will be designed to study the effect of intestinal disease (protozoa, bacteria) and drugs on enzyme level and the rate of its turnover.